taskflow/doc/source/states.rst

States

Note

The code contains explicit checks during transitions using the models described below. These checks ensure that a transition is valid, if the transition is determined to be invalid the transitioning code will raise a :py~taskflow.exceptions.InvalidState exception. This exception being triggered usually means there is some kind of bug in the code or some type of misuse/state violation is occurring, and should be reported as such.

Engine

RESUMING - Prepares flow & atoms to be resumed.

SCHEDULING - Schedules and submits atoms to be worked on.

WAITING - Wait for atoms to finish executing.

ANALYZING - Analyzes and processes result/s of atom completion.

SUCCESS - Completed successfully.

FAILURE - Completed unsuccessfully.

REVERTED - Reverting was induced and all atoms were not completed successfully.

SUSPENDED - Suspended while running.

UNDEFINED - Internal state.

GAME_OVER - Internal state.

Flow

PENDING - A flow starts its execution lifecycle in this state (it has no state prior to being ran by an engine, since flow(s) are just pattern(s) that define the semantics and ordering of their contents and flows gain state only when they are executed).

RUNNING - In this state the engine running a flow progresses through the flow.

SUCCESS - Transitioned to once all of the flows atoms have finished successfully.

REVERTED - Transitioned to once all of the flows atoms have been reverted successfully after a failure.

FAILURE - The engine will transition the flow to this state when it can not be reverted after a single failure or after multiple failures (greater than one failure may occur when running in parallel).

SUSPENDING - In the RUNNING state the engine running the flow can be suspended. When this happens, the engine attempts to transition the flow to the SUSPENDING state immediately. In that state the engine running the flow waits for running atoms to finish (since the engine can not preempt atoms that are actively running).

SUSPENDED - When no atoms are running and all results received so far have been saved, the engine transitions the flow from the SUSPENDING state to the SUSPENDED state.

Note

The engine may transition the flow to the SUCCESS state (from the SUSPENDING state) if all atoms were in fact running (and completed) before the suspension request was able to be honored (this is due to the lack of preemption) or to the REVERTED state if the engine was reverting and all atoms were reverted while the engine was waiting for running atoms to finish or to the FAILURE state if atoms were running or reverted and some of them had failed.

RESUMING - When the engine running a flow is interrupted 'in a hard way' (e.g. server crashed), it can be loaded from storage in any state (this is required since it is can not be known what state was last successfully saved). If the loaded state is not PENDING (aka, the flow was never ran) or SUCCESS, FAILURE or REVERTED (in which case the flow has already finished), the flow gets set to the RESUMING state for the short time period while it is being loaded from backend storage [a database, a filesystem...] (this transition is not shown on the diagram). When the flow is finally loaded, it goes to the SUSPENDED state.

From the SUCCESS, FAILURE or REVERTED states the flow can be ran again; therefore it is allowable to go back into the RUNNING state immediately. One of the possible use cases for this transition is to allow for alteration of a flow or flow details associated with a previously ran flow after the flow has finished, and client code wants to ensure that each atom from this new (potentially updated) flow has its chance to run.

Task

PENDING - A task starts its execution lifecycle in this state (it has no state prior to being ran by an engine, since tasks(s) are just objects that represent how to accomplish a piece of work). Once it has been transitioned to the PENDING state by the engine this means it can be executed immediately or if needed will wait for all of the atoms it depends on to complete.

Note

A engine running a task also transitions the task to the PENDING state after it was reverted and its containing flow was restarted or retried.

RUNNING - When an engine running the task starts to execute the task, the engine will transition the task to the RUNNING state, and the task will stay in this state until the tasks :py~taskflow.task.BaseTask.execute method returns.

SUCCESS - The engine running the task transitions the task to this state after the task has finished successfully (ie no exception/s were raised during execution).

FAILURE - The engine running the task transitions the task to this state after it has finished with an error.

REVERTING - The engine running a task transitions the task to this state when the containing flow the engine is running starts to revert and its :py~taskflow.task.BaseTask.revert method is called. Only tasks in the SUCCESS or FAILURE state can be reverted. If this method fails (ie raises an exception), the task goes to the FAILURE state (if it was already in the FAILURE state then this is a no-op).

REVERTED - A task that has been reverted appears in this state.

Retry

Note

A retry has the same states as a task and one additional state.

PENDING - A retry starts its execution lifecycle in this state (it has no state prior to being ran by an engine, since retry(s) are just objects that represent how to retry an associated flow). Once it has been transitioned to the PENDING state by the engine this means it can be executed immediately or if needed will wait for all of the atoms it depends on to complete (in the retry case the retry object will also be consulted when failures occur in the flow that the retry is associated with by consulting its :py~taskflow.retry.Decider.on_failure method).

Note

A engine running a retry also transitions the retry to the PENDING state after it was reverted and its associated flow was restarted or retried.

RUNNING - When a engine starts to execute the retry, the engine transitions the retry to the RUNNING state, and the retry stays in this state until its :py~taskflow.retry.Retry.execute method returns.

SUCCESS - The engine running the retry transitions it to this state after it was finished successfully (ie no exception/s were raised during execution).

FAILURE - The engine running the retry transitions it to this state after it has finished with an error.

REVERTING - The engine running the retry transitions to this state when the associated flow the engine is running starts to revert it and its :py~taskflow.retry.Retry.revert method is called. Only retries in SUCCESS or FAILURE state can be reverted. If this method fails (ie raises an exception), the retry goes to the FAILURE state (if it was already in the FAILURE state then this is a no-op).

REVERTED - A retry that has been reverted appears in this state.

RETRYING - If flow that is associated with the current retry was failed and reverted, the engine prepares the flow for the next run and transitions the retry to the RETRYING state.